The distal radioulnar joint is a "shallow socket" ball joint. The radius articulates in pronation and supination on the distal ulna. The ulna, a relatively straight forearm bone linked to the wrist, translates dorsal-palmarly to accept the modestly bowed radius. Since the sigmoid fossa socket in most wrists is relatively flat, ligaments are required to support the distal ulna. These ligaments include the triangular fibrocartilage (TFC), the extensor carpi ulnaris (ECU) subsheath, and the ulnar collateral ligament complex. The stabilizing elements of the triangular fibrocartilage (TFC), extensor carpi ulnaris (ECU) subsheath, and the ulnar collateral complex are well recognized along with the importance of a distal ulna component (ulnar head) for transfer of compressive loads between the ulnar carpus and the distal ulna across the distal radioulnar joint.
Ligament disruption, ulnar styloid fractures, and fractures into the distal radioulnar joint are common occurrences following fractures of the distal radius and other rotational instability injuries of the forearm. Fracture or dislocation involving the distal radioulnar joint often results in a loss of forearm rotation related to either instability or incongruity between the sigmoid fossa of the distal radius and the ulnar head. A variety of different fractures involving the distal radius can cause this condition including the Colles' fracture and the Galeazzi fractures.
When there is loss of stability of the distal radioulnar joint, there is subsequent weakness in grip and pinch as well as potential loss of forearm rotation. Instability can also be associated with either an injury to the triangular fibrocartilage or to the ulnar styloid. When instability is present, a number of ligament reconstructive procedures have been devised to assist in treating the unstable distal ulna. Unfortunately, ligament reconstruction of the distal ulna is often incomplete in restoring stability, and joint replacement is often necessary.
Where there is an incongruity of the joint surface involving either the articulation of the ulnar head with the sigmoid fossa of the distal radius, or if there is a significant ulnar impaction syndrome between the distal articular surface of the head of the ulna and the ulna carpus, a joint replacement may be necessary. Specifically, this can include either joint replacement of the distal ulna or operative procedures designed to shorten the ulna or resect all or part of the distal ulna (i.e. Darrach, Bowers, or matched resection procedures). Unfortunately, there have been variable results associated with the partial or complete resections of the distal ulna, particularly those performed by open resection. For example, when the ulna is resected, and not replaced with a prosthesis, both instability of the wrist and "snapping" of the forearm in rotational pronation/supination can occur.
The primary indications, therefore, for reconstruction of the distal radioulnar joint by prosthetic replacement (ulnar head replacement only) are generally related to a fracture of the distal ulna or a fracture extending into the distal radioulnar joint producing post-traumatic arthritis. Degenerative arthritis from other causes is also a primary indication. This is considered if there is associated arthritis and an ulnar shortening procedure is contraindicated. A third condition for primary ulna replacement is rheumatoid arthritis with a painful and unstable distal radioulnar joint. In these situations, prosthetic replacement of the distal ulna with soft tissue advancement can be beneficial.
A distal ulnar prosthesis is also suitable to correct a previous resection of the distal ulna that has failed. Such will be the case for 1) partial resection of the joint articular surface, as described by Feldon, Bowers, or Watson, or 2) complete resection of the distal ulna as recommended by Darrach, Baldwin, and others. When faced with failed distal ulna resection, one has options towards reconstruction without restoring the distal radioulnar joint (DRUJ). For example, a failed distal ulna can be corrected by a pronator quadratus interposition, or, if there has been only a partial resection, a fusion of the distal radioulnar joint combined with a proximal pseudarthrosis (Suave-Kapandji procedure). These procedures, however, do not restore the normal DRUJ function of motion or load transfer and may be associated with instability of the distal ulna and proximal impingement of the ulna on the distal radius. In these cases, a distal ulna prosthesis is generally preferable. A distal ulnar prosthesis is also suitable to correct a previous prosthetic replacement such as a silicone ulnar head replacement which has failed.
Unfortunately, attempts to create a suitable distal ulnar prosthesis that provides adequate stabilization and support have heretofore failed. Although biomechanics research studies have clearly demonstrated a need for prosthetic replacement of the distal ulna for load sharing across the carpus (Palmer et al, Berger), an adequate prosthesis for the distal ulna has heretofore been unavailable.
In light of the above it is an object of the present invention to provide a distal ulnar prosthesis which is attachable to a soft tissue pocket that includes the triangular fibrocartilage, ECU subsheath, and ulnar collateral ligament complex to thereby maintain distal radioulnar joint stability. It is another object of the present invention to provide a distal ulnar prosthesis which aligns anatomically with the sigmoid fossa of the distal radius and is isosymmetric with the anatomic center of rotation of the forearm. It is yet another object of the present invention to provide an anatomic distal ulnar prosthesis that duplicates the normal articulation of the distal ulna with the radius. It is yet another object of the present invention to provide a distal ulnar prosthesis that allows for a normal forearm rotation of approximately 150-170.degree.. Yet another object of the present invention is to provide a distal ulnar prosthesis which is relatively easy to implant, relatively simple to manufacture, and comparatively cost effective.